Fuse alarm systems



Jan. 28 1958 F. AmylmKs 2,821,697

FUSE ALARM SYSTEMS Filed May 29, 1956 EVE/V [Q] EVE/V w l EN TOR By E A.MIN/(S ATTORNEY United States Patent FUSE ALARM SYSTEMS Floyd A. Minks,Chester, N. H., assignor to Bell Telephone Laboratories, Incorporated,New York, N. Y., a corporation of New York Application May 29, 1956,Serial No. 588,106 6 Claims. (Cl. 340-250) This invention relatesgenerally to fuse alarm systems and more particularly, although in itsbroader aspects not exclusively, to fuse alarm systems for use inconnection with supplying direct operating potentials to the electrontube heater and anode circuits of voice and carrier frequency telephonerepeaters and terminal equipment.

In installations of telephone repeater and terminal equipment, it iscommon practice to use common direct voltage sources along with feederconductors or busses to energize electron tube heater circuits witheither 24 or 48 volts and electron tube anode circuits with +130 volts.In large installations, separate busses (termed ,odd and even) are oftenused to supply each potential to alternate bays or banks of equipmentand, in order to prevent the failure of one from removing from serviceall of the trunks between any two offices, a separate main feeder fuseis used for each bus. These main feeder fuses may, if desired, be of theso-called alarm type in order to provide an indication whenever one ormore of them has blown. In order to provide more localized protection,however, both heater and anode circuits or small groups of each areconnected to the respective feeder busses through individual fuses.These latter fuses are of the alarm type and, in the usual installation,a common alarm circuit is arranged to energize either a visible or anaudible signaling device or a combination of both in response to thefailure of any one or more of the individual alarm fuses.

Fuses of the so-called alarm type have been used in telephone equipmentinstallations for a good many years, a typical example of such a fusebeing disclosed in U. S. Patent 817,959, issued April 17, 1906, to E. B.Craft. When its fusible element fails, such a fuse operates not only todisconnect its load from the energized feeder bus but also to connectthe feeder bus to a separate alarm bus. In the usual installation, asuitable signaling device is connected between the alarm bus and groundand is operated whenever one or more of the individual alarm fusesfails.

While telephone repeater and terminal equipment fuse alarm systems ofthe type described have been found to work satisfactorily under mostconditions, it has been found that in some installations certaincombinations of individual alarm fuse and main feeder fuse failures tendto create fire hazards and marginal or unreliable operation of thesignaling device in the common fuse alarm circuit.

In some installations, for example, small resistors are used 2,821,697Patented Jan. 28, 1958 dividual circuit alarm fuses on both the heaterand the anode circuit feeder busses are connected to the same alarm bus,and the common alarm circuit signaling device is connected between thatalarm bus and ground. It has been found in these installations that, inthe event of simultaneous failure of both a heater and an anode circuitalarm fuse, the alarm currents through the common alarm circuitsignaling device tend to cancel one another and make uncertain theprompt operation of the signaling device.

One important object of the present invention is, therefore, to preventfire hazards and avoid circulating currents for all foreseeablecombinations of individual circuit alarm fuse and main feeder fusefailure in a fuse alarm system of the type described.

Another equally important object of the invention is to avoid thepossibility of marginal operation of the common alarm circuit signalingdevice in a fuse alarm system of the type described for all foreseeablecombinations of individ ual circuit alarm fuse failure.

In accordance with a first feature of the invention, the fire hazardsfound in some prior art fuse alarm systems for certain combinations ofindividual circuit alarm fuse and main feeder fuse failure areeliminated by the connections of asymmetrically conducting devices suchas diodes between each heater circuit alarm fuse and the alarm busleading from the heater circuit alarm fuses to the alarm circuitsignaling device. Each diode is poled to pass current from the heatercircuit feeder bus to the signaling device in the direction of normalflow when only the associated alarm fuse is operated. As a result, thealarm circuit path to the other heater circuit feeder bus and from therethrough other low impedance heater circuits to ground which would existupon simultaneous failure of a main feeder fuse and individual heatercircuit alarm fuses on both heater circuit feeder busses is blocked. Theentire alarm current is directed through the alarm circuit signalingdevice and possible fire hazards are avoided.

In accordance with a second important feature of the invention, themarginal alarm circuit operation occurring in some previous fuse alarmsystems for certain combinations of individual circuit alarm fusefailure is avoided with the aid of two instead of one alarm busses andat least two asymmetrically conducting devices such as diodes. Inaccordance with this feature of the invention, the individual heatercircuit alarm fuses and the individual anode circuit alarm fuses areconnected to separate alarm busses. The separate alarm busses areconnected to opposite sides of the common alarm circuit signalingdevice, and oppositely poled diodes are connected between the respectivesides of the signaling device and ground in order to ensure that alarmcurrents from all main feeder busses not only pass through the signalingdevice but also pass through in the same direction. As a result,reliable operation of the common alarm circuit signaling device isprovided whether a heater circuit alarm fuse, an anode circuit alarmfuse, or a combination of both, is blown.

Additional objects and features of the invention will become apparentupon study of the following detailed description of the specificembodiment of the invention illustrated in the single figure of thedrawing.

The embodiment of the invention illustrated in the drawing isspecifically adapted to supply direct operating potentials to theelectron tube heater and anode circuits of negative impedance voicefrequency telephone repeaters of the types shown in U. S. Patent2,742,616, issued April 17, 1956, to J. L. Merrill, Jr., and inapplication Serial No. 417,650, filed March 22, 1954, by S. T. Meyers.In the specific embodiment shown, the electron tube heater circuits aresupplied with a potential of -48 3 volts, while the electron tube anodecircuits are supplied with a potential of +130 volts.

The illustrated embodiment of the invention includes a pair ,of heatercircuit feeder busses 1 and 2 and a pair of anode circuit feeder busses3 and 4. Feeder busses 1 and 3 are labeled odd to indicate that theysupply the direct operating potentials to the repeaters in odd numberedgroups, while feeder busses 2 and 4 are labeled even to indicate thatthey perform the same function for even numbered groups. Feeder busses 1and 2 are joined at one end and energized with a -48 volt potential, asillustrated diagrammatically in the drawing by a battery 5 connectedbetween the feeder busses and ground. Feeder busses 3 and 4 aresimilarly joined and energized with a +130 volt potential, as showndiagrammatically by a battery 6 connected between the busses and ground.Each feeder bus is separately fused in order, as explained above, toprevent the failure of one from removing from service all of the trunksbetween any two central ofiices. As illustrated, a first pair of mainfeeder fuses 7 and 8 are connected in series with feeder busses 1 and 2,respectively, and a second pair of main feeder fuses 9 and 10 areconnected in series with feeder busses 3 and 4, respectively. Mainfeeder fuses 7 through 10 may themselves be of the alarm type, althoughthis has no immediate bearing on the operation of the remainder of theillustrated fuse alarm system.

By way of example, the drawing shows electron tube heater and anodecircuits for two repeaters in an odd group .and for two repeaters in aneven group. The odd group repeaters include a pair of electron tubes 11and 12 and a pair of electron tubes 13 and '14, respectively, while theeven group repeaters include a pair of electron tubes and 16 and a pairof electron tubes 17 and 18, respectively. Tubes 11 and 12 contain apair of respective heaters 19 and 20, tubes 13 and 14 contain a pair ofrespective heaters 21 and 22, tubes 15 and 16 contain a pair ofrespective heaters 23 and 24, and tubes 17 and 18 contain a pair ofrespective heaters 25 and 26.

In the illustrated embodiment of the invention, heaters 19 and 20 areconnected to ground from odd heater circuit feeder bus 1 through anindividual alarm fuse 27. Heaters 21 and 22 are similarly connectedthrough an alarm fuse 2 8. Heaters 23 and 24 and heaters 25 and 26 arereturned to ground from even heater circuit feeder bus 2 throughrespective alarm fuses 29 and 30. Each heater combination has aresistance of the order of 100 ohms or less. The impedances presented bythe various vacuum tube anode circuits are much greater, however, andtherefore do not provide extraneous paths to ground for alarm currentsunder any possible fuse conditions. For this reason, the anode circuitsare shown only in sketchy fashion, with no complete showing of impedanceelements other than the tubes themselves in the paths from theindividual anode circuit alarm fuses to ground. Tubes 11 and 12 andtubes 13 and 14 are, however, supplied from odd anode circuit feeder bus3 through alarm fuses 31 and 32, respectively, and tubes 15 and '16 andtubes 17 and 18 are supplied from even anode circuit feeder bus 4through alarm fuses 33 and 34, respectively.

As has already been briefly explained, each of the individual circuitalarm fuses 27 through 34 operates, when the fusible element fails, todisconnect its load from the feeder bus and to connect the feeder bus toa separate alarm circuit bus. The illustrated embodiment of theinvention includes two separate alarm busses 35 and 36. In accordancewith an important feature of the invention, the alarm electrodes offuses 27 through are connected directly to alarm bus through respectivediodes 37 through 40, all of which are poled for easy current flow fromalarm bus 35 toward their respective alarm fuses. Alarm bus 35 is, inturn, connected to one side of the operating coil of an alarm circuitrelay 41.v The relay 4 operating coil is shunted by an alarm circuitlamp 42 and its other side is connected to the reference potential,ground, through a diode 43. Diode 43 is poled for easy current flow inthe direction from ground toward the operating coil of relay 41.

in accordance with another important feature of the invention, the alarmelectrodes of alarm fuses 31 through 34 are all connected to the otheralarm bus 36. A pair of small resistors 44 and 45 are connected betweenalarm bus 36 and alarm fuses 31 and '33, respectively, in order tominimize the circulating currents which tend to occur if the voltagedrops on the anode circuit feeder busses 3 and 4 should be unequal.Alarm bus 36 is connected through a larger resistor 46 and a diode 47 tothe side of the operating coil of relay 41 opposite from alarm bus 35,and a diode 48 is connected between the alarm bus 35 side of theoperating coil and ground. Diode 47 is poled for easy current flow fromresistor 46 toward the operating coil of relay 41 and diode 48 is poledfor easy current flow from the operating coil toward ground. Thearmature of relay 41 is connected to open and close an alarm circuit 49which may, if desired, contain audible signaling devices, more visiblesignaling devices, or a combination of both.

The operating advantages made possible by the various features of theinvention may be understood most clearly if the illustrated embodimentis contrasted from the prior art. Consider, for example, the situationexisting if diodes 37 through 40 were not connected between alarm bus 35and alarm fuses 27 through 30 and simple resistors were so connectedinstead. If main feeder fuse 7 should be blown or removed and if heatercircuit alarm fuses 27 and 29 were to blow simultaneously, a relativelylow impedance path to ground would exist from battery 5 through mainfeeder fuse 8, feeder bus 2, alarm fuse 29, the resistor taking theplace of diode 39, the resistor taking the place of diode 37, alarm fuse27, feeder bus 1, alarm fuse 28, and the low impedance represented byvacuum tube heaters 21 and 22 to ground. Since the impedance of theoperating coil of relay 41 is of the order of 2000 ohms and that of lamp42 is even higher, most of the alarm current would flow through theresistors and heaters 21 and 22 to ground. As a result of theseexcessive currents, the resistors would most likely be destroyed and apossible fire hazard would be created.

The present invention eliminates such fire hazards in a comparativelysimple and inexpensive manner. Consider the situation existing in theillustrated embodiment of the invention if the same fuses were blown.Electrons (electron flow is less awkward to trace than current flowbecause of the polarity of battery 5) would flow from battery 5 throughmain feeder fuse 8, feeder bus 2, alarm fuse 29, diode 39, lamp 42 andthe operating coil ofrelay 41, and diode 43 to ground. Diode 37 preventselectrons from getting through to alarm fuse 21 and feeder bus 1 forreturn to ground through alarm fuse 28 and the low impedance pathprovided by heaters 21 and 22. In this manner, alarm currents areconfined to their intended paths and possible fire hazards are avoided,regardless .of the combination of main feeder fuses and heater circuitalarm fuses blown.

The other principal advantage over the prior art afforded by theinvention may also be best appreciated if the illustrated embodiment iscontrasted from the prior art. Consider, for example, the situationobtaining if, as is usual, anode circuit alarm fuses 31 through 34 wereconnected to alarm bus 35 instead of to alarm bus 36 and diode 43 wereshorted out. Alarm bus 35 could then be energized by either the heatercircuit feeder busses 1 and 2 or the anode circuit feeder busses 3 and4. As long as a heater circuit alarm fuse did not blow simultaneouslywith an anode circuit alarm fuse, operation would be normal, withcurrent from the feeder bus associated with the blown alarm fuseenergizing alarm bus 35, lamp 42, and alarm relay 41. If, however, aheater circuit alarm fuse and an anode circuit alarm fuse (such as alarmfuses 27 and 31, respectively) should blow simultaneously, the

operation of alarm relay 41 would be only marginal. The alarm currentsfrom oppositely poled batteries and 6 would oppose each other in theoperating coil of relay 41 and, if relay 41 would operate at all, itwould operate with a considerably weaker action. If the voltage drop inthe feeder bus associated with the blown anode circuit alarm fuse shouldbe sufliciently great, the two alarm currents could even cancel eachother to an extent sufficient to keep relay 41 from operating at all andto keep lamp 42 from burning with anything but a very dim light.

The present invention not only prevents such marginal or unreliablealarm circuit operation when individual circuit alarm fuses fail on bothheater circuit and alarm circuit feeder busses, but also makes operationstronger and more afiirmative. In accordance with a principal feature ofthe invention, the two alarm busses 35 and 36 are com nected to oppositesides of the operating coil of relay 41 and diodes 43 and 48 areconnected to insure that alarm currents from both sets of feeder bussesnot only pass through the coil but also pass through it in the samedirection. If, for example, alarm fuses 27 and 31 were blown in theillustrated embodiment of the invention, electrons from battery 5 wouldpass through main feeder fuse 7, heater circuit feeder bus 1, alarm fuse27, diode 3'7, lamp 42 and the operating coil of relay 41, and diode 43to ground. The current flow through lamp 42 and the operating coil ofrelay 41 would be from bottom to top in the drawing. The polarity ofdiode 4S prevents any electrons from battery 5 from leaking off toground before reaching lamp 42 and relay 41. Current from battery 6, onthe other hand, would flow through main feeder fuse 9, anode circuitfeeder bus 3, alarm fuse 31, resistor 44, resistor 46, diode 47, lamp 42and the operating coil of relay 41, and diode 48 to ground. As before,the current flow through lamp 42 and the operating coil of relay 41would be from bottom to top in the drawing. The polarity of diode 43:prevents current from battery 6 from reaching ground before coming tolamp 42 and relay 41.

The embodiment of the invention shown in the drawing is thussubstantially foolproof. Undesirable and possibly dangerous currents areblocked by the configuration of diodes 37 through 40 and the arrangementof alarm busses 35 and 36 and diodes 43, 47, and 48 forces alarmcurrents always to traverse lamp 42 and the operating coil of relay 41in the same direction, regardless of the number and combination of blownfuses.

What is claimed is:

1. In combination, a pair of feeder busses each supplying a directpotential of one polarity to a plurality of low impedance loads, a pairof main feeder fuses connected in series with respective ones of saidpair of feeder busses and carrying the combined currents of the loadsassociated therewith, a third feeder bus supplying a direct potential ofthe opposite polarity to a plurality of other loads, a separate alarmfuse connected between each of said loads and the assocaited one of saidfeeder busses, each of said alarm fuses being adapted when blown todisconnect its lead from the feeder bus and connect the feeder bus to analarm bus, a common alarm circuit for all of said alarm fuses includingat least one signaling device and a pair of alarm busses, a first ofsaid alarm busses being connected between the alarm fuses associatedwith said pair of feeder busses and one side of said signaling deviceand the second of said alarm busses being connected between the alarmfuses associated with said third feeder bus and the other side of saidsignaling device, oppositely poled asymmetrically conducting devicesconnected between respective sides of said signaling device and a pointof predetermined reference potential to ensure that alarm currents fromall of said feeder busses not only pass through said signaling devicebut also pass through in the same direction, and a separateasymmetrically conducting device connected between each alarm fuseassociated with said pair of feeder busses and said first alarm bus toprevent alarm currents from flowing through some of said low impedanceloads when one of said pair of main feeder fuses is open and an alarmfuse is blown on each of said pair of feeder busses.

2. In combination, a pair of feeder busses, a first source of directpotential connected between each of said pair of feeder busses and apoint of predetermined reference potential, a plurality of low impedanceloads connected between each of said pair of feeder busses and saidpoint of reference potential, a pair of main feeder fuses connected inseries with respective ones of said pair of feeder busses between saidfirst source of direct potential and the respective low impedance loads,a third feeder bus, a second source of direct potential opposite inpolarity to said first source connected between said third feeder busand said point of reference potential, a plurality of other loadsconnected between said third feeder bus and said point of referencepotential, a separate alarm fuse connected between each of said loadsand the associated one of said feeder busses, each of said alarm fusesbeing adapted when blown to disconnect its load from the feeder bus andconnect the feeder bus to an alarm bus, a common alarm circuit for allof said alarm fuses including at least one signaling device and a pairof alarm busses, a first of said alarm busses being connected betweenthe alarm fuses associated with said pair of feeder busses and one sideof said signaling device and the second of said alarm busses beingconnected between the alarm fuses associated with said third feeder busand the other side of said signaling device, oppositely poledasymmetrically conducting devices connected between respective sides ofsaid signaling device and said point of reference potential to ensurethat alarm currents from both of said sources not only pass through saidsignaling device but also pass through in the same direction, and aseparate asymmetrically conducting device connected between each alarmfuse associated with said pair of feeder busses and said first alarm busto prevent alarm currents from flowing through some of said lowimpedance loads when one of said pair of main feeder fuses is open andan alarm fuse is blown on each of said pair of feeder busses.

3. In combination, a first feeder bus supplying a direct potential ofone polarity to a plurality of loads, a second feeder bus supplying adirect potential of the opposite polarity to a plurality of other loads,a separate alarm fuse connected between each of said loads and theassociated one of said feeder busses, each of said alarm fuses beingadapted when blown to disconnect its load from the feeder bus and toconnect the feeder bus to an alarm bus, a common alarm circuit for allof said alarm fuses including at least one signaling device and a pairof alarm busses, one of said alarm busses being connected between thealarm fuses associated with said first feeder bus and one side of saidsignaling device and the other of said alarm busses being connectedbetween the alarm fuses associated with said second feeder bus and theother side of said signaling device, and oppositely poled asymmetricallyconducting devices connected between respective sides of said signalingdevice and a point of predetermined reference potential to ensure thatalarm currents from both of said feeder busses not only pass throughsaid signaling device but also pass through it in the same direction.

4. In combination, a first feeder bus, a first source of directpotential connected between said first feeder bus and a point ofpredetermined reference potential, a plurality of loads connectedbetween said first feeder bus and said point of reference potential, asecond feeder bus, a second source of direct potential opposite inpolarity to said first source connected between said second feeder busand said point of reference potential, a plurality of other loadsconnected between said second feeder bus and said point of referencepotential, a separate alarm fuse connected between each of said loadsand the associated one of said feeder busses, each of said alarm fusesbeing adapted when blown to disconnect its load from the feeder bus andto connect the feeder bus to an alarm bus, a common alarm circuit forall of said alarm fuses including at least one signaling device and apair of alarm busses, one of said alarm busses being connected betweenthe alarm fuses associated with said first feeder bus and one side ofsaid signaling device and the other of said alarm busses being connectedbetween the alarm fuses associated with said second feeder bus and theother side of said signaling device, and oppositely poled asymmetricallyconducting devices connected between respective sides of said signalingdevice and said point of reference potential to ensure that alarmcurrents from both of said sources not only pass through said signalingdevice but also pass through in the same direction.

5. In combination, a pair of feeder busses supplying substantially thesame direct potential to respective low impedance loads, a pair of mainfeeder fuses connected in series with respective ones of said feederbusses and carrying the combined currents for the loads associatedtherewith, a separate alarm fuse connected between each of said loadsand the associated one of said feeder busses, each of said alarm fusesbeing adapted when blown to disconnect its load from the feeder bus andto connect the feeder bus to an alarm bus, a common alarm circuit forall of said alarm fuses including at least one signaling device and analarm bus, said alarm bus being connected between said alarm fuses andOne side of said signaling device and the other side of said signalingdevice being connected to a point of predetermined reference potential,and a separate asymmetrically conducting device connected between eachalarm fuse and said alarm bus to prevent alarm currents from flowingthrough some of said low impedance loads when one of "said pair offeeder fuses is open and an alarm fuse is blown-on each of said feederbusses.

6. In combination, a pair of feeder busses, a source of direct potentialconnected between each of said feeder busses and a point ofpredetermined reference potential, a plurality of low impedance loadsconnected between each of said feeder busses and said point of referencepotential, a pair of main feeder fuses connected in series withrespective ones of said feeder busses between said source of directpotential and the respective said low impedance loads, a separate alarmfuse connected be tween each of said loads and the associated one ofsaid feeder busses, each of said alarm fuses being adapted when blown todisconnect its load from the feeder bus and to connect the feeder bus toan alarm bus, a common alarm circuit for all of said alarm fusesincluding at least one signaling device and an alarm bus, said alarm busbeing connected between said alarm fuses and one side of said signalingdevice and the other side of said signaling device being connected tosaid point of reference potential, and a separate asymmetricallyconducting device connected between each alarm fuse and said alarm busto prevent alarm currents from flowing through some of said lowimpedance loads when one of said pair of main feeder fuses is open andan alarm fuse is blown on each of said feeder busses.

References Cited in the file of this patent UNITED STATES PATENTS2,329,518 Dimond Sept, 14, 1943 2,464,848 Collins Mar. 22, 1949 2,769,970 Kratville Nov. 6, 1956

